A variety of materials may be used to block sound, but no single material blocks noise, which is sound over a full range of high to low audible frequencies, without using relatively massive amounts of the single material.
What is commonly referred to as “noise” nearly always is the combination of many different frequencies. By way of example, the sound spectrum analysis presented in FIG. 1 depicts a 3-cylinder diesel engine running at 2,800 RPMs. The “noise” it emits includes frequencies ranging from 22 Hz to over 11 kHz. No single material can effectively block all these frequencies, at least without utilizing large amounts of that material, which may be impractical in many instances, such as onboard aircraft, seacraft, and/or automobiles, etc.
Moreover, materials that do block low audible frequencies, and, perhaps to a slightly lesser extent, medium audible frequencies (as opposed to the higher frequencies), tend to be very dense, and thus heavy. In comparison, foam, which has a resonant frequency that is comparatively quite high (above 250-500 Hz depending on thickness), allows lower frequencies to pass through virtually unaffected, only blocking higher frequencies. Attenuating these lower frequencies is difficult (more difficult than attenuating the higher frequencies). Lead and filled (also known as mineral loaded) vinyl of similar density, which relies on a higher mass to dampen the lower frequencies, may be used. Unfortunately, the material's comparatively heavy weight makes the use of lead and the like prohibitive in many applications (e.g., aircraft, seacraft, etc.) Moreover, these materials rely on the phenomenon of sound waves causing the material to vibrate when the waves impinge on the material. Due to the weight of the material, this vibration creates a new problem if the vibrating mass is allowed to mechanically transfer energy to another surface (such as a wall or bulkhead). In such an instance, the noise is simply transferred through and radiated off the other surface.
Conventional acoustic insulation materials used to address lower frequencies work well if they are of a high weight and thickness. There remains a need in the art for lighter and thinner damping layers that effectively attenuate noise in mid to low frequency ranges.